The long-range goal of the proposed research is to identify by mutation all the indispensable genes in the small soil nematode Caenorhabditis elegans and to investigate microscopically the developmental consequences of mutation in each gene. C. elegans has been chosen for this work because of its suitability for genetical methods and its relative cellular simplicity. The specific objectives of this proposal are: first, to isolate genetically marked chromosome translocations or inversions that can be used as balancers to maintain recessive lethal or recessive sterile mutations in heterozygotes and to facilitate their subsequent manipulation; second, to saturate at least one small region of the genome, covered by one balancer, with recessive lethal and recessive sterile mutations induced by ethyl methansulfonate and to assign the mutations to genes by complementation; and third, to categorize any detectable defects in cell lineage in the mutants by light microscopy. Screening for translocations and inversions is to be carried out by classical methods adapted to C. elegans' hermaphroditic mode of reproduction: X-irradiation followed by selection for loss of linkage of markers or suppression of appearance of cross-over products.